From prior art is known the use of an HVDC network for transmission of electric power over long distances. An HVDC network consists of a rectifier station, a transmission line in the form of a cable or an overhead line, a station for inversion and one or more filters for eliminating harmonics generated during the inversion. In a common embodiment of an HVDC network, also known as classic HVDC, non-extinguishable semiconducting elements such as for instance thyristors are used both for rectification and inversion. Thyristors are capable of being fired but not capable of being extinguished. The commutation takes place near the zero crossing of the voltage, which zero crossing is determined by the ac voltage and the inverters. These converters are therefore referred to as line-commutated and also known as current source converters (CSC). The technique involves the inversion to consume reactive power and leads to harmonics sent out onto the network. In the following, converters which are used as rectifier or as inverter will be referred to as converters.
From prior art is known to arrange the HVDC transmission network with overhead lines. Such uninsulated overhead lines have been used to implement a direct voltage network interconnecting stations which may be located a very large distance apart, for example, 1000 km or more. However, such overhead lines have a significant disturbing influence upon the country and the environment. The alternative has been to use cables. Suitable cables comprise an inner conductor surrounded by a thick insulating layer formed by a paper impregnated by oil. These cable, however, are so costly that they are not a realistic alternative to overhead lines. Experiments have been carried out with cables having a conductor and an insulating layer on a polymer base surrounding the conductor. These experiments have revealed great problems in using this type of cable for HVDC transmission, since cavities are created in the insulating layer. A cavity causes a deformation of the electric field in the cable which leads to partial discharges, also known as space charges. These partial discharges create gases that attack the polymeric material. In a matter of time the insulating layer has been broken down and the cable has lost its insulating performance. The space charges are caused by the changes of polarity which the conductor is subjected to when the transmission direction of the effective power is changed. It is thus known to arrange overhead lines for transmitting HVDC on land, and cables having tailor made oil-impregnated insulating sheets at sea for electric power transmission between land areas separated by water.
From U.S. Pat. No. 5,910,888 (Wiemers) a plant for transmitting electric power is previously known. The object of the plant is to avoid the problems of cable breakdown in combination with HVDC. Hence, a Plant for transmission of HVDC is provided comprising voltage source converters (VSC). These converters are known not the change the polarity of the DC lines when the direction of the effective power transmission is changed.
Still there is a need within the HVDC technique industry to be able to produce a more cost effective classic HVDC using cables.